Optical parametric oscillator with distributed feedback grating or distributed Bragg reflector

ABSTRACT

The invention relates to a simple and effective laser device for generating laser radiation from an optical parametric oscillator (OPO) by implementing a distributed feedback (DFB) or distributed Bragg reflector (DBR) structure, in the laser oscillator. The nonlinear crystal used in the OPO of the present invention can be a birefringence phase-matched crystal or a quasi-phase-matched crystal.

FIELD OF THE INVENTION

[0001] The present invention relates to a laser device, and more particularly to a laser device for generating laser radiation from an optical parametric oscillator (OPO) by using a Bragg diffraction structure in or nearby a nonlinear optical gain medium.

BACKGROUND OF THE INVENTION

[0002] An optical Bragg diffraction grating discussed in the present invention is an optical grating reflecting an optical wavelength satisfying the Bragg condition λ_(m)=2nΛ_(g)/m, where n is the refractive index seen by the optical wave, Λ_(g) is the spatial wavelength of the grating, and m is a positive integer. The wavelength λ_(m) is called the Bragg wavelength of order m. Therefore a Bragg diffraction grating can be used as an optical filter, like a fiber grating. When a Bragg diffraction grating is fabricated at the two ends of a laser gain medium to replace the resonator mirrors of a laser, the laser is often called a distributed Bragg reflector (DBR) laser. A DBR laser has the advantage of simplicity, because the two physical resonator mirrors are replaced by two Bragg diffraction gratings monolithically fabricated to a laser gain medium. In a laser, a Bragg diffraction grating can be fabricated over the whole length of a laser gain medium. The distributed optical feedback from each grating element establishes laser oscillation near the Bragg wavelength. This type of laser is termed distributed-feedback (DFB) laser. Since different longitudinal modes in a DFB laser have different laser gain, a DFB laser has the advantage of simplicity and single-longitudinal-mode operation.

[0003] The DBR and DFB structures have been widely used in diode lasers for a variety of applications. However the wavelength of a diode laser is limited by the energy band gap of a semiconductor material, and cannot be varied with an arbitrary choice of the DBR or DFB grating period.

[0004] One way to tune a laser wavelength is to use a nonlinear optical material. Since 1962, the quasi-phase-matched (QPM) nonlinear frequency conversion technique has been described by J. A. Armstrong et al. in Phys. Rev. 127 (1962) 1918. Solid-state laser pumped optical parametric generation (OPG) and oscillation from, for instance, periodically-poled lithium niobate (PPLN) has provided efficient and widely tunable laser sources (Myers et al. Journal of Optical Society of America B, vol. 12 (1995) pp. 2102-2116). However, optical parametric generation produces a broadband radiation. In the mid-infrared wavelengths, the spectral width of PPLN OPG may exceed several nanometers. To obtain efficient narrow-line laser radiations, QPM optical parametric oscillation is a popular means. A conventional linear-cavity optical parametric oscillator (OPO) may take advantage of the high finesse from its resonator and generate multi-longitudinal-mode radiation with a narrow linewidth in each mode. To obtain single longitudinal mode, one usually has to adopt a more complicated resonator design (Bosenberg et al., Applied Physics Letters, vol. 61 (1992) pp. 378-389). Yet if one could implement a DBR or DFB structure into the nonlinear optical medium of an optical parametric oscillator, one would obtain any wavelength in the tuning range of the oscillator. In particular, a DFB structure in the nonlinear optical medium permits single-frequency oscillation at a chosen wavelength.

[0005] It is therefore attempted by the applicant to deal with the above situation encountered with the prior art.

SUMMARY OF THE INVENTION

[0006] It is therefore an object of the present invention to propose a simple and effective laser device for generating laser radiation from an optical parametric oscillator by implementing a Bragg diffraction grating in the nonlinear optical medium. However, the nonlinear optical material used in the OPO of the present invention can be a birefringence phase-matched crystal or a quasi-phase-matched crystal.

[0007] It is therefore another object of the present invention to propose a simple and effective laser device for generating a narrow-spectral-width laser from an optical parametric oscillator by implementing a distributed feedback grating (DFB) or distributed Bragg reflectors (DBR) in the nonlinear optical medium. However, the nonlinear optical material used in the OPO of the present invention can be a birefringence phase-matched crystal or a quasi-phase-matched crystal.

[0008] It is an aspect of the present invention to propose a laser device for generating narrow-spectral-width laser from an optical parametric oscillator (OPO) with a nonlinear optical element having a photorefractive DFB grating for oscillating an electromagnetic wave at one of the parametric mixing wavelengths.

[0009] It is another aspect of the present invention to propose a laser device for generating narrow-spectral-width laser from an optical parametric oscillator (OPO) with a nonlinear optical element having a photorefractive DBR structure for oscillating an electromagnetic wave at one of the parametric mixing wavelengths.

[0010] Preferably, the DFB or the DBR structure reflects the electromagnetic wave near the first-order Bragg wavelength λ=2nΛ_(g), where Λ_(g) is a Bragg diffraction grating period and n is the effective refractive index seen by the Bragg scattered wave in the medium, so that the laser device with the DFB or DBR structure oscillates at the wavelength λ. The Bragg wavelength λ can be any of the nonlinear mixing wavelengths.

[0011] Preferably, the nonlinear optical element is a second-order nonlinear optical crystal.

[0012] Preferably, the OPO has a built-in photorefractive DFB structure in a bulk nonlinear crystal.

[0013] Preferably, the OPO has a built-in photorefractive DFB structure in a waveguide nonlinear crystal.

[0014] Preferably, the OPO has a built-in photorefractive DBR structure in a bulk nonlinear crystal.

[0015] Preferably, the OPO has a built-in photorefractive DBR structure in a waveguide nonlinear crystal.

[0016] Preferably, the nonlinear optical crystal is a birefringence phase-matched crystal.

[0017] Preferably, the nonlinear optical crystal is a quasi-phase-matched crystal.

[0018] Preferably, the nonlinear optical crystal is a photorefractive nonlinear optical crystal such as lithium niobate or lithium tantalate or impurity-doped same.

[0019] It is a further aspect of the present invention to propose a laser device for generating narrow-spectral-width laser from an optical parametric oscillator (OPO) with a nonlinear optical element having a photorefractive DFB or DBR structure wherein the photorefractive index modulation is fabricated by shining an optical beam through a grating mask atop the photorefractive nonlinear optical element.

[0020] Preferably, the grating mask has a grating period matching the Bragg condition desirable for the OPO operation.

[0021] Preferably, the optical beam is an optical write beam having the effect of inducing space charges in the photorefractive nonlinear optical material.

[0022] Preferably, the optical beam is at the visible or UV wavelengths.

[0023] Preferably, the nonlinear optical element is nonlinear optical crystal having the second order susceptibility.

[0024] It is an additional aspect of the present invention to propose a laser device for generating narrow-spectral-width laser from an optical parametric oscillator (OPO) with a nonlinear optical element having a photorefractive DFB or DBR structure wherein the photorefractive index modulation is fabricated by an interferometric photorefractive writing scheme, with which two crossed laser beams with a proper crossing angle introduce spatial optical intensity modulation in the nonlinear optical element through the interference effect.

[0025] Preferably, the periodicity of the interference fringes matches the Bragg condition desirable for the OPO operation.

[0026] It is another aspect of the present invention to propose a laser device for generating narrow-spectral-width laser from an optical parametric oscillator (OPO) with a nonlinear optical element having an electro-optic DFB grating, wherein an electric field is applied to the electro-optic nonlinear optical element for spatially varying the refractive index in the laser propagation direction. The periodicity of the refractive index variation matches the Bragg condition desirable for the OPO operation.

[0027] It is another aspect of the present invention to propose a laser device for generating narrow-spectral-width laser from an optical parametric oscillator (OPO) with a nonlinear optical element having an electro-optic DBR grating, wherein an electric field is applied to the electro-optic nonlinear optical element for spatially varying the refractive index in the laser propagation direction. The periodicity of the refractive index variation matches the Bragg condition desirable for the OPO operation.

[0028] Preferably, the amount of index change, in its simplified form, is given by Δn=½rn³E, where r is an electro-optic coefficient of the crystal, n is the refractive index without the applied voltage, and E is an electric field inside the crystal.

[0029] It is another aspect of the present invention to propose a laser device for generating narrow-spectral-width laser from an optical parametric oscillator (OPO) having a DFB structure atop a nonlinear optical waveguide, with which the distributed feedback from the evanescent waves on the waveguide surface is sufficient for starting optical parametric oscillation.

[0030] Preferably, the DFB structure is fabricated by using schemes such as material etching, or thin film coating, micro-lithography, and a combination of the same.

[0031] It is another aspect of the present invention to propose a laser device for generating narrow-spectral-width laser from an optical parametric oscillator (OPO) having a DBR structure atop a nonlinear optical waveguide, with which the distributed feedback from the evanescent waves on the waveguide surface is sufficient for starting optical parametric oscillation.

[0032] Preferably, the DBR structure is fabricated by using schemes such as material etching, or thin film coating, micro-lithography, and a combination of the same.

[0033] It is another aspect of the present invention to propose a laser device for generating narrow-spectral-width laser from an optical parametric oscillator (OPO) having a DFB or DBR structure inside a nonlinear optical waveguide, with which the optical feedback from the Bragg scattered waves inside the waveguide is sufficient for starting optical parametric oscillation.

[0034] Preferably, the DFB or DBR structure is of photorefractive type mentioned above.

[0035] Preferably, the DFB or DBR structure is of electro-optic type mentioned above.

[0036] The present invention may best be understood through the following description with reference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0037]FIG. 1 is a schematic cross-sectional view of an OPO, as practiced in the prior art.

[0038]FIG. 2 is a schematic illustration of an embodiment of the OPO of the invention.

[0039]FIG. 3 is a schematic view of a DFB structure in a nonlinear optical bulk material used in the OPO of the invention.

[0040]FIG. 4 is a schematic view of the DBR structure in a nonlinear optical bulk material used in the OPO of the invention.

[0041]FIG. 5 is a schematic view of a DFB structure in a nonlinear optical waveguide used in the OPO of the invention.

[0042]FIG. 6 is a schematic view of the DBR structure in a nonlinear optical waveguide used in the OPO of the invention.

[0043]FIG. 7 is a schematic view of a chirped Bragg diffraction grating as another preferred embodiment of the DFB or DBR structure for the nonlinear optical material used in the present invention.

[0044]FIG. 8 is a schematic view of a cascaded Bragg diffraction grating as another preferred embodiment of the DFB or DBR structure for the nonlinear optical material used in the present invention.

[0045]FIG. 9 is the schematic of a photorefractive DFB OPO by using a UV photomask to write the Bragg diffraction grating in a photorefractive nonlinear optical crystal.

[0046]FIG. 10 is the measured signal spectra from the photorefractive DFB OPO in FIG. 9.

[0047]FIG. 11 is the schematic of the two-photon photorefractive DFB OPO by using crossed laser beams to write the Bragg diffraction grating in a photorefractive nonlinear optical crystal.

[0048]FIG. 12 is the measured signal spectra from the two-photon photorefractive DFB OPO in FIG. 11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0049] Although this invention is susceptible to embodiments of many different forms, some preferred embodiments would be described and illustrated in detail hereinafter. The present disclosure exemplifies the principle of the invention and is not to be considered a limitation to the broader aspects of the invention to the particular embodiment as described.

[0050] This invention relies on the optical feedback from a Bragg diffraction grating to establish the optical parametric oscillation in an optical parametric oscillator. FIG. 1 illustrates the prior art where a pump laser 10 pumps a nonlinear optical element 30 in a laser resonator formed by two reflecting mirrors 20. In an optical parametric process, the pump photon with angular frequency cop generates two photons with angular frequencies ω_(s) and ω_(i) according to the relationship

ω_(s)+ω_(i)=ω_(p).

[0051] Usually the high frequency photon is called the signal and the low frequency one is called the idler. The resonator mirrors 20 may oscillate the signal or the idler photon or both.

[0052] According to the present invention, the resonator mirrors in the prior art are replaced by a DFB or DBR structure in the nonlinear optical element, as illustrated in FIG. 2. The nonlinear optical element with a built-in DFB or DBR structure 40 can be any of the devices 41, 42, 43, 44, 45, 46 in FIGS. 3-8.

[0053]FIG. 3 is a schematic view of a DFB structure in a nonlinear optical bulk material used in the OPO of the invention. There exist a periodic refractive index modulation, designated by the repeating labeling 2 and 3, over the whole length of the nonlinear optical bulk material. The refractive index modulation period or the Bragg grating period of order m, Λ_(g,m), satisfies the Bragg condition ${\Lambda_{g,m} = \frac{m\quad \lambda_{s}}{2n_{s}}},{{{or}\quad \Lambda_{g,m}} = \frac{m\quad \lambda_{i}}{2n_{i}}},$

[0054] where λ_(s) and λ_(i) are the signal and idler wavelengths, respectively, and n_(s) and n_(i) are the effective refractive indices seen the signal and idler photon, respectively.

[0055]FIG. 4 is a schematic view of the DBR structure in a nonlinear optical bulk material used in the OPO of the invention. There exist two sections of the Bragg diffraction gratings 50 at the two ends of a nonlinear optical bulk material to serve as the reflecting mirrors in the OPO of the present invention. A section of a nonlinear optical material 55 between the two Bragg diffraction gratings is left unchanged for optical frequency conversion.

[0056]FIG. 5 is a schematic view of a DFB structure in a nonlinear optical waveguide used in the OPO of the invention. There exist a corrugated periodic structure 80, the DFB structure, atop a nonlinear optical waveguide 60 over the whole device length. The DFB structure reflects the evanescent wave on the waveguide surface to establish the optical parametric oscillation inside the waveguide. The nonlinear optical waveguide is fabricated on the surface of a suitable material substrate 70. For example, a nonlinear optical waveguide can be fabricated on a lithium niobate substrate by the so-called annealed proton-exchanged method (M. L. Bortz et al. Optics Letters vol. 16 No. 23 (1991) pp. 1844-1846). An alternate embodiment of the Bragg diffraction grating is to have the periodic refractive index change embedded inside the waveguide. The embedded DFB structure thus provides distributed optical feedbacks to the optical wave inside the waveguide directly.

[0057]FIG. 6 is a schematic view of a DBR structure in a nonlinear optical waveguide used in the OPO of the invention. Instead of having the Bragg diffraction grating over the whole length of the device, two sections of the Bragg diffraction gratings 50, the DBR structure, are fabricated atop or inside a nonlinear optical waveguide at the two ends of the device. The nonlinear optical waveguide 55 between the DBR reflectors is left untouched.

[0058]FIG. 7 is a schematic view of a chirped Bragg diffraction grating as another preferred embodiment of the DFB or DBR structure for the nonlinear optical element used in the present invention. A general Bragg diffraction grating may have an arbitrary refractive index variation in space, which reflects the optical wavelengths matching the spatial Fourier components in the Bragg diffraction grating through the Bragg condition. Therefore the reflection spectrum of a Bragg diffraction grating can be mapped to the Fourier transform spectrum of the Bragg diffraction grating. A chirped Bragg diffraction grating has a broad Fourier transform spectrum and therefore is suitable for broadband OPO operation.

[0059]FIG. 8 is a schematic view of a cascaded Bragg grating as another preferred embodiment of the DFB or DBR structure for the nonlinear optical element used in the present invention. The cascaded Bragg diffraction grating is suitable for generating several laser wavelengths from the OPO simultaneously as each Bragg grating (90, 95, . . . , 99), having a different grating period, oscillates a different wavelength in the OPO. As long the all the Bragg wavelengths in the cascaded Bragg grating are within the OPO bandwidth, the OPO of the present invention may generate a plural number of laser wavelengths simultaneously.

[0060] When a photorefractive nonlinear optical element is used as the gain medium of an OPO, it is possible to write a photorefractive Bragg diffraction grating in the bulk or waveguide element. In a photorefractive material, space charges are established inside the material with a spatial modulation matching the intensity of a write beam. Coupled through the electro-optical effect, the refractive index of the nonlinear optical material is spatially modulated according to the spatially intensity variation in the write beam. For example, a spatially intensity modulated write beam can be implemented by using photomask atop the photorefractive nonlinear optical element with a desired Bragg-diffraction-grating period in the photomask. Alternatively, a phase mask can be used to interfere the write beam with an interference-intensity period equal to the desired Bragg-diffraction-grating period. Furthermore, one can simply cross two laser beams with an appropriate angle to form an periodic interference intensity in the photorefractive nonlinear optical crystal to write the photorefractive Bragg diffraction grating.

[0061] When an electro-optic nonlinear crystal is used as the gain medium of an OPO, it is possible to implement an electro-optic Bragg diffraction grating in the bulk or waveguide crystal. One may initially fabricate micro-electrodes on the nonlinear crystal with an electrode distribution matching the Bragg structure distribution by using the micro-lithographic techniques. When a voltage is applied to the micro-electrodes, the electric field strength in the crystal induces refractive index change through the electro-optic effect to form an electro-optic Bragg diffraction grating in the crystal. For example, if a 0.5 mm thick, z-cut periodically poled lithium niobate (PPLN) crystal is used for a quasi-phase-matching OPO, one may fabricate the periodic micro-electrodes to the crystallographic ±z surface and apply a voltage to form a Bragg diffraction grating. According to the equation ${{\Delta \quad n} = {\frac{1}{2}r_{33}n^{3}E_{z}}},$

[0062] the index change can be on the order of Δn=10⁻⁵ when a voltage of ˜100 Volts is applied to the micro-electrodes.

[0063] If a nonlinear optical waveguide is used for an OPO, the Bragg diffraction grating can be fabricated on the waveguide surface by using lithographic etching process. The micro-etching process often used in the semiconductor processing forms a corrugated Bragg diffraction grating on the nonlinear waveguide. Like a DFB or DBR diode laser, the Bragg reflection from the evanescent wave is sufficient to establish the optical parametric oscillation. Alternatively, the Bragg diffraction grating can be formed by thin-film coating and lithographic etching techniques atop the nonlinear waveguide. For example, one may initially coat a layer of material thin film to the nonlinear waveguide and apply chemical or plasma etching to the thin film to obtain a Bragg diffraction grating on the nonlinear waveguide.

[0064] Experiment

[0065] Since lithium niobate is a photorefractive material, it is possible to write a photorefractive DFB or DBR structure in a PPLN crystal. In particular, with the OPO mirrors replaced by a photorefractive DFB structure in a PPLN crystal, the design of a single-frequency OPO can be greatly simplified. In this experiment, we show two PPLN DFB optical parametric oscillators with photorefractive DFB gratings in PPLN crystals. One photorefractive DFB grating was fabricated by illuminating ultraviolet (UV) light through a photomask atop the PPLN crystal, and the other was written into the PPLN crystal by interfering laser beams at 532-nm wavelength.

[0066] In the UV-photomask scheme shown in FIG. 9, incoherent UV radiation from a 20W, mercury lamp was illuminated onto a photomask with a 1-μm period, 50%-duty-cycle chromium grating mask. The mercury lamp was covered with a UV filter that is transparent to the 365-nm mercury line. With the UV filter, the incident intensity on the photomask was about 0.3 W/cm². The photomask was in contact with a 4-cm long, 0.5-mm thick, 28-μm-period, uncoated and end-polished PPLN crystal with the chromium grating vector aligned with the QPM grating vector in the crystallographic x direction. While illuminating UV radiation through the photomask, we raised the PPLN crystal temperature from 20° C. to 160° C. within 3 minutes and decreased the temperature from 160° C. to 20° C. over a 2-hour duration. The UV-induced photorefractive DFB grating was thus fixed in the PPLN. The 1-μm DFB grating period allows the oscillation of the 4.085-μm idler wavelength in the 1064-nm pumped PPLN OPO at 115.4° C. temperature. The corresponding signal wavelength is 1438.8 nm. Pumping the DFB PPLN with a 9-μJ/pulse, 730-ps pulse width passively Q-switched Nd:YAG laser, we observed that the DFB OPO signal was generated at 1438.8 nm. The signal spectra were measured by using an InGaAs detector after a ½-m grating monochromator. The resolution of the monochromator was 3 Å with a 10-μm slit opening and a 300 lines/mm infrared grating. The 730-ps pump pulse length is comparable to the round-trip time in the 4-cm long PPLN crystal, and does not establish parametric oscillation from the uncoated PPLN end faces.

[0067]FIG. 10 shows the OPO and the OPG signal spectra at different temperatures. It is evident from FIG. 10 that, although the OPG wavelength was shifted by temperature, the OPO signal wavelength remained unchanged due to the photorefractive DFB grating in the PPLN. At 115.4° C., the OPG wavelength overlapped the OPO signal wavelength, and the conversion was significantly enhanced by a factor of 3. The measured spectral width of the DFB OPO signal was 3 Å, and that of the OPG signal was 3 nm. At the pump energy of 6.75 μJ, the output signal energy was 1 μJ, corresponding to 15% signal conversion efficiency.

[0068] With the DFB grating vector in the crystallographic x direction, one would expect that the DFB grating results from the space charge field in the x direction E_(x). However, the E_(x)-induced refractive index change for a z-polarized pump field is a second-order effect in lithium niobate, given by

Δn _(z) =n ₀ ² n _(e) ³(r ₅₁ E _(x))²/2,   (1)

[0069] where n₀ and n_(e)≈2.2 are the ordinary and extraordinary refractive indices respectively, and r₅₁≈32 pm/V is the electro-optic coefficient of lithium niobate. In the steady-state, thermal-diffusion-dominated, and nondepleted-carrier approximation, the space charge field is about the order of E_(x)≈2πk_(B)T/(Λq)≈10⁵ V/M, where k_(B) is Boltzmann's constant, q is the electron charge, T≈400 K is the crystal temperature, and Λ=1 μm is the DFB grating period. Although this space charge field rotates the index ellipsoid of lithium niobate and slightly depolarizes the incident optical field, the index change seen by the laser is merely Λn_(z)≈10⁻⁹, according to Eq. (1). However due to UV diffraction through the photomask, the intensity of the write beam spatially varies along the crystallographic z direction. This suggests that the space charge distribution also varies in the z direction. With the photorefractive charges varying along z, a z-component space charge field E_(z) may induce the first-order refractive-index change in PPLN, given by

Δn _(z) =n _(e) ³ r ₃₃ E _(z)/2≈10⁻⁵,   (2)

[0070] wherein we have used r₃₃≈31 pm/V and E_(z)≈E_(x)≈10⁵ V/m for calculation. This amount of refractive-index change is much larger than that calculated from Eq. (1). The distributed optical feedback in our PPLN OPO was similar to that in a DFB diode laser with a corrugated grating at the top portion of the gain medium. Nonetheless, the corrugated photorefractive DFB grating was able to provide sufficient optical feedbacks and produce a signal spectral linewidth reaching the limit of the monochromator resolution.

[0071] To further investigate the possibility of a DFB OPO by using the x-direction space charge field in PPLN, we tried to write the photorefractive DFB grating by using interfering laser beams at 532-nm wavelength, as shown in FIG. 11. We used a cylindrical lens to shape the spherical laser beam into an elliptical beam with approximately 1:50 axis ratio. Then we split and recombined the 200-mW, 532-nm laser with a 34° angle to produce 0.913-μm-period interference fringes on a 5-cm long, 0.5-mm thick, 11-μm-period, uncoated and end-polished PPLN crystal. The peak intensity of the interfering laser beam was about 0.5 W/Cm². The 0.913-μm-period DFB grating was designed to oscillate the 3.778-μm OPO idler wavelength in the 532-nm pumped PPLN at 82° C. The pump laser was a passively Q-switched, frequency-doubled Nd:YAG laser, producing 2-μJ pulse energy in the PPLN crystal with 6.59-kHz repetition rat e and 430-ps pulse width. The interfering 532-nm laser beams produced a periodic space charge field in the x direction in the bulk of the PPLN crystal. Although the temperature dependent OPG signal near 620 nm was clearly seen in our experiment, we did not observe any evidence of DFB OPO due to the trivial photorefractive index change given by Eq. (1).

[0072] While keeping the 532-nm interference fringes in the PPLN crystal, we illuminated the PPLN +z surface with UV light of the intensity 53 mW/cm² and observed again the DFB OPO signal at the output. The UV light at the 365-nm wavelength was attenuated along the z direction in the lithium niobate and induced a corrugated DFB structure through the so-called two-photon photorefractive writing scheme (L. Hesselink, et al., Science 282 (1998) pp. 1089). FIG. 12 shows the OPO and OPG signal spectra from the PPLN at different temperatures. It is evident from FIG. 12 that, although the OPG wavelength was tuned by temperature, the DFB OPO signal wavelength at 619.3 nm again remained unchanged due to the photorefractive DFB grating. At 82.4° C., the OPG wavelength overlapped with the OPO signal wavelength. When we translated the pump beam transversely in the −z direction, the DFB OPO signal was reduced gradually due to the spatial variation of the photorefractive DFB grating in the z direction. In this experiment, the DFB OPO signal spectral width is also 3 Å.

[0073] Since the photorefractive effect, relying on the electro-optic effect, falls into the category of an electro-optic phenomenon, the experiments mentioned above can be generalized to the electro-optic DFB or DBR structure for use in the OPO of the present invention.

[0074] Typically the refractive index change from the photo-refractive effect and the electro-optic effect is small compared to the proposed etched or coated DFB or DBR structure. Therefore it is a straightforward generalization from the above experiments that an etched or coated DFB or DBR structure in an OPO nonlinear optical element also falls into the scope of this invention. 

What is claimed is:
 1. A laser device for generating laser radiation from an optical parametric oscillator (OPO) with a nonlinear optical element having a built-in Bragg diffraction grating for oscillating electromagnetic waves within the bandwidth of said OPO.
 2. The laser device according to claims 1, wherein said Bragg diffraction grating is a DFB structure having a grating period matching the Bragg condition for establishing single-wavelength laser oscillation in said OPO.
 3. The laser device according to claims 1, wherein said Bragg diffraction grating is a DBR structure having a grating period matching the Bragg condition for establishing narrow-band laser oscillation in said OPO.
 4. The laser device according to claims 1, wherein said Bragg diffraction grating is a chirped grating structure having a broad-band spatial frequency components matching the Bragg condition for establishing broad-band laser oscillation in said OPO.
 5. The laser device according to claims 1, wherein said Bragg diffraction grating is a cascaded grating structure having multiple grating periods matching the Bragg condition for establishing multi-wavelength laser oscillation in said OPO.
 6. A laser device for generating narrow-spectral-width laser from an optical parametric oscillator (OPO) with a nonlinear optical element having a photorefractive DFB structure for oscillating an electromagnetic wave at one of the parametric mixing wavelengths.
 7. A laser device for generating narrow-spectral-width laser from an optical parametric oscillator (OPO) with a nonlinear optical element having a photorefractive DBR structure for oscillating an electromagnetic wave at one of the parametric mixing wavelengths.
 8. The laser device according to claims 6 or 7, wherein said DFB or said DBR structure has a spatial periodicity Λ_(g) satisfying the Bragg condition Λ_(g)=mλ/2n, where n is the effective refractive index seen by the Bragg wavelength λ, and m is a positive integer, so that said laser device oscillates at the wavelength λ.
 9. The laser device according to claims 6 or 7, wherein said Bragg wavelength λ is any of the nonlinear mixing wavelengths.
 10. The laser device according to claims 1, 6 or 7, wherein said nonlinear optical element is a nonlinear optical crystal having second-order nonlinear susceptibility.
 11. The laser device according to claims 6, wherein said OPO has a built-in photorefractive DFB structure in a bulk nonlinear crystal.
 12. The laser device according to claims 6, wherein said OPO has a built-in photorefractive DFB structure in a waveguide nonlinear crystal.
 13. The laser device according to claims 7, wherein said OPO has a built-in photorefractive DBR structure in a bulk nonlinear crystal.
 14. The laser device according to claims 7, wherein said OPO has a built-in photorefractive DBR structure in a waveguide nonlinear crystal.
 15. The laser device according to claim 10, wherein said second-order nonlinear optical crystal is a birefringence phase-matched crystal.
 16. The laser device according to claim 10, wherein said second-order nonlinear optical crystal is a quasi-phase-matched crystal.
 17. The laser device according to claim 10, wherein said second-order nonlinear optical crystal is a photorefractive nonlinear optical crystal selected from the material group consisting of lithium niobate, lithium tantalate and impurity-doped same.
 18. A laser device for generating narrow-spectral-width laser from an optical parametric oscillator (OPO) with a nonlinear optical element having one of the photorefractive DFB and DBR structures in which the photorefractive index variation is fabricated by shining an optical beam through a grating mask atop the photorefractive nonlinear optical element.
 19. The laser device according to claim 18, wherein said grating mask has a grating period matching the Bragg condition desirable for said OPO operation.
 20. The laser device according to claim 18, wherein said optical beam is an optical write beam having the effect of inducing space charges in the photorefractive nonlinear optical material.
 21. The laser device according to claim 18, wherein said optical beam is at one of visible and UV wavelengths.
 22. The laser device according to claim 18, wherein said nonlinear optical element is a nonlinear optical crystal having the second-order nonlinear susceptibility.
 23. A laser device for generating narrow-spectral-width laser from an optical parametric oscillator (OPO) with a nonlinear optical element has one of the photorefractive DFB and DBR structures wherein the photorefractive index variation is fabricated by an interferometric photorefractive writing scheme, with which two crossed laser beams with a proper crossing angle provides spatial optical intensity modulation in a nonlinear optical element through the interference effect.
 24. The laser device according to claim 23, wherein the periodicity of the interference fringes matches the Bragg condition desirable for said OPO operation.
 25. A laser device for generating narrow-spectral-width laser from an optical parametric oscillator (OPO) with an electro-optic nonlinear optical element having an electro-optic DFB structure, wherein an spatially varying electric field is applied to said electro-optic nonlinear optical element for spatially varying the refractive index in the laser propagation direction and the periodicity of said refractive index variation matches the Bragg condition desirable for said OPO operation.
 26. A laser device for generating narrow-spectral-width laser from an optical parametric oscillator (OPO) with an electro-optic nonlinear optical element having an electro-optic DBR structure, wherein an spatially varying electric field is applied to said electro-optic nonlinear optical element for spatially varying the refractive index in the laser propagation direction and the periodicity of said refractive index variation matches the Bragg condition desirable for said OPO operation.
 27. A laser device for generating narrow-spectral-width laser from an optical parametric oscillator (OPO) having a DFB structure atop a nonlinear optical waveguide, with which the distributed optical feedback from the evanescent waves on a waveguide surface is sufficient for starting optical parametric oscillation.
 28. The laser device according to claim 27, wherein said DFB structure is fabricated by using schemes selected from the group consisting of material etching, material implantation, thin film coating, micro-lithography, and a combination of the same.
 29. A laser device for generating narrow-spectral-width laser from an optical parametric oscillator (OPO) having a DBR structure atop a nonlinear optical waveguide, with which the distributed optical feedback from the evanescent waves on a waveguide surface is sufficient for starting optical parametric oscillation.
 30. The laser device according to claim 29, wherein the DBR structure is fabricated by using schemes selected from the group consisting of material etching, material implantation, thin film coating, micro-lithography, and a combination of the same.
 31. A laser device for generating narrow-spectral-width laser from an optical parametric oscillator (OPO) having one of the DFB and DBR structures inside a nonlinear optical waveguide, with which an optical feedback from Bragg scattered waves inside a waveguide is sufficient for starting optical parametric oscillation.
 32. The laser device according to claim 31, wherein one of said DFB and DBR structure is of photorefractive type.
 33. The laser device according to claim 31, wherein one of said DFB and DBR structure is of electro-optic type. 